1. Field of Invention
The present invention relates to a method for controlling parallel-connected frequency converters or inverter units feeding an alternating-current motor provided with parallel windings or separate parallel motors having their shafts mechanically coupled together via a load. The invention also relates to an arrangement for parallel connection of frequency converters or inverter units.
2. Description of Related Art
In high-power motors it is possible to use several parallel windings which are galvanically separate from each other and which can be connected in a desired manner. The windings be mounted at least partly in the same slots, in which case a strong magnetic coupling exists between the windings. The stator of the motor may also be divided into separate segments, so that it consists of several separate stators whose windings have no mutual magnetic coupling. The motor may be either an asynchronous or a synchronous motor.
In a prior-art arrangement for feeding a high-power multi-winding motor by a frequency converter, the frequency converter consists of several lower-power units (either separate complete frequency converters or inverter units connected to a common DC supply) which generate their output voltages in synchronism with each other. Therefore, each edge of the output voltage is as simultaneous as possible, which is achieved by an arrangement whereby a common control unit distributes the control pulses to the semiconductor switches, such as insulated gate bipolar transistors (IGBT), simultaneously to each parallel-connected frequency converter or inverter unit.
However, due to the non-ideal properties of the system (differences in signal transit times, different switching times of IGBT switches, differences in windings, etc.), the currents of the parallel-connected units are not necessarily evenly distributed. From this it follows that the system has to be designed according to the worst situation, which means that the achievable maximum power of the system is lower than the sum of the nominal powers of the parallel-connected units. To smooth out the current differences and thus to increase the total power, it is possible to add smoothing inductors between the frequency converters and the motor, which has the disadvantage of increasing the costs.